The present invention is directed generally to an improved method for the production of, methanol, acetic acid, and other chemicals such as vinyl acetate monomer (VAM) from natural gas. The improved method integrates a carbon monoxide separation plant with a methanol synthesis unit to form an optimal syngas composition for methanol production.
Methanol is a major chemical raw material. Major uses of methanol include the production of acetic acid, formaldehyde and methyl-t-butylether. Worldwide demand for methanol is expected to grow in the next decade as new applications become commercialized such as the conversion of methanol to gas (Mobil MTG Process), the conversion of methanol to light olefins (MTO Process of UOP and Norsk Hydro), the use of methanol for power generation and the use of methanol in fuel cells. The development of such applications is clearly linked to the methanol production cost. The present invention permits the construction of highly efficient single-train plants for converting natural gas to methanol at low cost in large quantities.
The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Pat. Nos. 1,961,736 to Carlin et al (Tennessee Products); 3,769,329 to Paulik et al (Monsanto); 5,155,261 to Marston et al (Reilly Industries); 5,672,743 to Garland et al (BP Chemicals); 5,728,871 to Joensen et al (Haldor Topsoe); 5,773,642 to Denis et al (Acetex Chimie); 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); 5,877,347 and 5,877,348 to Ditzel et al (BP Chemicals); 5,883,289 to Denis et al (Acetex Chimie); and 5,883,295 to Sunley et al (BP Chemicals), each of which is hereby incorporated herein by reference.
The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. For this reason, attention has recently focused on the construction of integrated plants producing both methanol and acetic acid. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or at least significantly reduced.
The primary raw materials for vinyl acetate monomer manufacture are ethylene, acetic acid and oxygen. Carbon dioxide is produced as an undesirable byproduct in the reaction and must be removed from the recycled ethylene.
A significant expense of new production capacity for syngas, methanol, acetic acid and acetic acid derivatives such as VAM, is the capital cost of the necessary equipment. Other significant expenses include the operating costs, including the cost of raw materials. It would be extremely desirable if these capital and operating costs could be reduced.
For methanol production, it is well established that for a large capacity syngas plant autothermal reforming could be the more economic process leading to synthesis gas, since large capital costs are saved by not constructing large primary reformers or multiple partial oxidation reformers. Nevertheless, the drawback is not being able to have a full usage of all carbon molecules, resulting in the venting of large quantities of CO2, which is undesirable. It is in fact necessary to condition the synthesis gas at the outlet of the autothermal reformer because the stoichiometric number (SN)=[(H2-CO2)/(CO+CO2)] is below 2, usually between 1.7 and 1.9. The goal is to obtain an optimum syngas ratio, which lies in the range of 2.0 to 2.1 for makeup to the methanol synthesis loop.
Lee et al discloses in U.S. Pat. No. 5,180,570 an integrated process for making methanol and ammonia in order to approach stoichiometric conditions in the methanol reaction loop. McShea, III et al disclose in U.S. Pat. No. 4,927,857 a catalyst for autothermal reforming and the means to obtain a syngas in stoichiometric proportions by controlling the steam to carbon and oxygen to carbon ratios. Supp et al disclose in U.S. Pat. No. 5,310,506 the addition of a high-hydrogen gas in the ATR feed to obtain, a synthesis gas exiting the ART suitable for methanol synthesis having a stoichiometric number of between 1.97 and 2.2. Banquy discloses in U.S. Pat. Nos. 4,888,130 and 4,999,133, a process suitable for methanol production on a very large scale where the synthesis gas can be made as close as necessary to the stoichiometric composition required for methanol production, by using the combination of both a primary steam reformer and an autothermal reactor.
In an article presented to 2000 World Methanol Conference Copenhagen Denmark Nov. 8-10, 2000, Streb shows that very large capacity methanol plants require a special process design. He suggests that pure autothermal reforming can be used when the feedstock is light natural gas, but he underlines that then the stoichiometric ratio is less than 2 and suggests the need to suppress CO2 conversion. In EP Patent Application No. 1,348,685 A1, Grobys et al disclose a process for the production of methanol wherein the syngas number is adjusted by withdrawing a carbon monoxide stream. In commonly assigned WO 03/097523A2, the present applicant discloses an integrated process that produces both methanol and acetic acid under substantially stoichiometric conditions.
In U.S. Pat. No. 6,495,609, Searle discloses the recycle of CO2 to a methanol synthesis reactor in the production of ethylene and ethylene oxide from methanol. In U.S. Pat. No. 6,444,712, Janda discloses the recycle of CO2 back to either the reformer or the methanol synthesis loops to control the SN between 1.6 and 2.1. Both Searle and Janda demonstrate the manipulation of the SN through the use of steam and partial oxidation reformers. Generally steam reformers generate syngas with an SN greater than 2.8, while partial oxidation reformers produce syngas having an SN between 1.4 and 2.1.
The rising need for hydrogen in refineries is driven by the increasingly stringent fuel specifications in terms of the content of aromatics and sulfur in gasoline and diesel. The importation of large quantities of hydrogen is necessary as hydrogen demand peaks and balances in refineries are jeopardized.